Field of the Invention
This invention relates generally to micro-reaction chamber electrodes and more particularly to micro-reaction chamber electrodes for neural stimulation and recording.
Description of the Prior Art
A major challenge for electrical interface to biological systems, especially neural interfaces, is that charge passing in biological tissue is through ions, whereas in electronic instrumentation is through electrons. Therefore, interfaces between instrumentation and tissue are limited by their ability to exchange charge. This is typically done through combinations of capacitance charge build-up and electrochemical reactions.
In general, electrochemistry deals with the processes that take place at the interface between the electronic conductor (electrode surface) and the ionic conductor (electrolyte). Electrochemical activity and hence the impedance of a particular electrode is restricted to the active area that is in contact with the electrolyte. Generally, only those materials at the exposed surface take part in the electrochemical processes making the underlying bulk substrate less important in charge passing as long as the surface coating is intact and defect free. Electrolyte-based electrochemical reactants and reaction products can undergo subsequent reactions in the bulk that affect both charge passing efficacy as well as safety.
In the case of neural interfaces, successful neural prosthesis requires efficient communication to and from central and/or peripheral nervous systems. Neural recording and stimulation electrodes act as transducers that mediate signal transport between the ionic tissue environment and the solid-state electronic environment of the prosthetic device. Electrodes of smaller geometry are generally preferred to improve the spatial locality and to decrease the tissue damage resulting from insertion trauma. This, however, leads to increase in interfacial impedance and increase in the required charge transfer density for a given stimulation pulse. Since charge transfer takes place at the electrode-tissue interface by either Faradaic or capacitive mechanisms, the two-dimensional interfacial area, also called the electrochemical surface area (ESA), determines the electrochemical activity of the electrodes. For an electrode with a given geometric surface area (GSA), improving the surface roughness either by etching the surface or by depositing porous coatings on the surface such as Pt black, iridium oxide, or conductive polymer helps increase ESA and enhance electrochemical activity of the electrodes. Modifying the surface morphology of the surface coating using micro and nanoscale templates to introduce pores has also resulted in significant increase in ESA. However, the useful thickness of these coatings is limited by the chemical transport inside the pores and the possibility of fragile surface coatings, cracking or delaminating under mechanical stress in situ.
It is therefore desirable to provide a solution to the electrochemical charge transfer limitation problem, and related reaction-product tissue damage, associated with current electrode designs.
It is further desirable to provide a micro-reaction chamber electrode having impedance that can approach that of an ideal, geometrically defined electrode independent of capacitive or Faradic effects.
It is further desirable to provide a micro-reaction chamber electrode having minimal electrode impedance, maximum charge passing capacity, improved reversibility, decreased tissue damage, and a longer operational life.